It is common practice to perform ignition timing control, i.e., mount a knocking sensor on an engine and control an ignition timing of the engine based on an output of the knocking sensor for suppression of engine knocking by proper control of engine operations (see Patent Document 1).
More specifically, an ignition signal indicative of the ignition timing is outputted from an electronic control unit to an ignitor. Based on this ignition signal, a control signal is outputted from the ignitor to an ignition coil so as to cause a spark plug to generate a spark discharge.
In the ignition timing control, a maximum advance value of the ignition timing allowable for the engine is set; and a reference ignition timing (reference ignition signal timing) is set, as a reference of the ignition timing, by a predetermined margin on a retard side with respect to the maximum advance value as shown in FIG. 12A.
When the occurrence of engine knocking is not detected by the knocking sensor, the ignition timing (ignition control signal timing) is advanced stepwisely. The ignition timing is retarded when the occurrence of engine knocking is detected by the knocking sensor. The occurrence of engine knocking is prevented by the adoption of such ignition timing control.